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我正在尝试使用来自 DBpedia 的 Lookup Web 服务，但我遇到了困难，因为网络上的信息很少。我找到了一个代码示例，但是当我执行它时，我得到了错误：

java.lang.NullPointerException
    at tutorial.DBpediaLookupClient.containsSearchTerms(DBpediaLookupClient.java:111)
    at tutorial.DBpediaLookupClient.endElement(DBpediaLookupClient.java:87)
    at org.apache.xerces.parsers.AbstractSAXParser.endElement(Unknown Source)
    at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanEndElement(Unknown Source)
    at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl$FragmentContentDispatcher.dispatch(Unknown Source)
    at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
    at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
    at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
    at org.apache.xerces.parsers.XMLParser.parse(Unknown Source)
    at org.apache.xerces.parsers.AbstractSAXParser.parse(Unknown Source)
    at org.apache.xerces.jaxp.SAXParserImpl$JAXPSAXParser.parse(Unknown Source)
    at org.apache.xerces.jaxp.SAXParserImpl.parse(Unknown Source)
    at javax.xml.parsers.SAXParser.parse(SAXParser.java:195)
    at tutorial.DBpediaLookupClient.<init>(DBpediaLookupClient.java:56)
    at tutorial.DBpediaLookupClient.main(DBpediaLookupClient.java:126)

这是代码：

package tutorial;

import java.io.IOException;
import java.io.InputStream;
import java.net.URLEncoder;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.StringTokenizer;
import javax.xml.parsers.SAXParser;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.parsers.SAXParserFactory;
import org.apache.commons.httpclient.HttpClient;
import org.apache.commons.httpclient.HttpException;
import org.apache.commons.httpclient.HttpMethod;
import org.apache.commons.httpclient.methods.GetMethod;
import org.xml.sax.SAXException;
import org.xml.sax.XMLReader;
import org.xml.sax.helpers.DefaultHandler;
import com.sun.org.apache.bcel.internal.classfile.Attribute;

public class DBpediaLookupClient extends DefaultHandler {

    /**
     * @param args
     */

    private List<Map<String, String>> variableBindings = new ArrayList<Map<String, String>>();
    private Map<String, String> tempBinding = null;
    private String lastElementName = null;
    private String query = "";

        public DBpediaLookupClient( String query ) throws Exception {
            this.query = query;
            HttpClient client = new HttpClient();
            HttpMethod method = new GetMethod("http://lookup.dbpedia.org/api/search.asmx/KeywordSearch?QueryClass=place&QueryString="+query);

            try{
                 client.executeMethod(method);
                System.out.println(method);

                InputStream ins = method.getResponseBodyAsStream();

                SAXParserFactory factory = SAXParserFactory.newInstance();
                javax.xml.parsers.SAXParser sax = factory.newSAXParser();
                sax.parse(ins, this);


            }catch (HttpException he) {
                System.err.println("Http error connecting to lookup.dbpedia.org");
            }catch (IOException ioe) {
                System.err.println("Unable to connect to lookup.dbpedia.org");
            }catch (ParserConfigurationException e) {
                System.out.println("O parser não foi configurado corretamente.");
                e.printStackTrace();
            }catch (SAXException e) {
                System.out.println("Problema ao fazer o parse do arquivo.");
                e.printStackTrace();
            }

                method.releaseConnection();
            }


        public void startElement(String uri, String localName, String qName, Attribute attributes) throws SAXException {
            if (qName.equalsIgnoreCase("result")) {
              tempBinding = new HashMap<String, String>();
            }
            lastElementName = qName;
          }



          public void endElement(String uri, String localName, String qName) throws SAXException {
            if (qName.equalsIgnoreCase("result")) {
                System.out.println("Qname:" + qName);
              if (!variableBindings.contains(tempBinding) && containsSearchTerms(tempBinding))
                variableBindings.add(tempBinding);
            }
          }



          public void characters(char[] ch, int start, int length) throws SAXException {
            String s = new String(ch, start, length).trim();

            if (s.length() > 0) {
              if ("Description".equals(lastElementName)) tempBinding.put("Description", s);
              if ("URI".equals(lastElementName)) tempBinding.put("URI", s);
              if ("Label".equals(lastElementName)) tempBinding.put("Label", s);
            }
          }


          public List<Map<String, String>> variableBindings() {
            return variableBindings;
          }



          private boolean containsSearchTerms(Map<String, String> bindings) {
            StringBuilder sb = new StringBuilder();

            for (String value : bindings.values()){
                sb.append(value);  // do not need white space
            }
            String text = sb.toString().toLowerCase();
            StringTokenizer st = new StringTokenizer(this.query);
            while (st.hasMoreTokens()) {
              if (text.indexOf(st.nextToken().toLowerCase()) == -1) {
                return false;
              }
            }
            return true;
          }



          public static void main(String[] args) {
              try {
                DBpediaLookupClient client = new DBpediaLookupClient("berlin"); 

            } catch (Exception e) {
                e.printStackTrace();
            }
          }

}

如果有人知道出了什么问题，请帮助我。或者，如果你们中的任何人有任何其他代码示例，它也会有很大帮助。

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2 回答 2

0

我与代码的作者交谈，他纠正了一些小错误。这是代码：

这是更正代码的链接：https ://github.com/mark-watson/java_practical_semantic_web/blob/master/src/com/knowledgebooks/info_spiders/DBpediaLookupClient.java

这是将 Dbpedia Lookup Service 与 Jena 一起使用的一个非常好的示例。它完美地工作！

于 2013-07-23T12:54:42.247 回答
0

请注意 SAX paserhandler，它可能会在字符方法中为您提供错误的长度，因此请重新构建链接的 java 代码，如下所示：

private StringBuffer fechaiBuffer;
public void startElement(...) {
  fechaiBuffer = new StringBuffer();
}
public void characters(char ch[], int start, int length) {
  fechaiBuffer.append(ch, start, length);
}
public void endElement(...) {
  s = fechaiBuffer.toString();
  // and so on
}
于 2015-03-09T15:56:42.103 回答